[Linux] show/hide emission hooks are never removed; verify clean exit

Start in src/platform/linux/window_manager_linux.cpp at InstallGlobalSwizzling(), comparing the show/hide hooks with the nearby focus-in/focus-out hooks. Trace where window_manager shuts down and verify the hook lifecycle there. Done means a nativeapi Flutter app exits cleanly on Linux under X11 and Wayland, including after closing a secondary window created through package:nativeapi/windowing.dart.

InstallGlobalSwizzling() in src/platform/linux/window_manager_linux.cpp adds show / hide emission hooks on GtkWidget with g_signal_add_emission_hook(), but doesn't keep the hook ids, so they are never removed. The focus-in/focus-out hooks next to them are removed properly.

window_manager had a similar never-removed button-press-event emission hook in its Linux plugin. Users reported crashes when the app exits, or when a secondary Flutter window closes: FlutterEngineRemoveView ... The implicit view cannot be removed, followed by an epoxy/GLX assertion. The hooks here pass no user data, so they probably can't use a freed plugin pointer, but that should be confirmed:

• keep the hook ids and remove them when the manager shuts down
• check that a Flutter app with nativeapi exits cleanly on Linux (X11 and Wayland), including after a secondary window created through package:nativeapi/windowing.dart is closed

Background:

• leanflutter/window_manager#585: crash on exit on Fedora 44 KDE X11, from just having the plugin
• leanflutter/window_manager#567: a PR that removes the emission hook on plugin dispose